Method and apparatus for fully digital beam formation in a phased array coherent imaging system

ABSTRACT

A method for steering a beam of vibratory energy to a desired angle θ with respect to the plane of an array of transducers, uses apparatus which generates a plurality of sampling strobe sequences with each strobe pulse occurring at a time interval T after the preceding strobe pulse, where T satisfies the Nyquist condition. A different one S j  of the strobe signals is assigned to each different transducer channel and the commencement time of the strobe signal in each channel is offset by a time interval which is a first positive integer multiple M j , selected for each angle θ, of an offset time interval which is not greater than 1/32nd of the reciprocal of the ultrasonic excitation frequency F u . Each strobe signal triggers conversion of the present amplitude of the return signal directly to a digital data word; the apparatus then digitally delays each data word in each of the N channels for a delay time interval selected to cause the delayed data words from all N channels, when coherently summed, to represent any change in reflectance in the beam at the desired angle θ.

This application is a continuation-in-part of U.S. patent applicationSer. No. 921,516, filed on Oct. 22, 1986 and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to coherent imaging systems usingvibratory energy, such as ultrasonic or electromagnetic waves, and, moreparticularly, to a novel method for forming the vibratory (ultrasonic)beam, including beam direction (steering), focussing and apodizationfunctions, totally by digital (rather than analog) signal processing ofthe vibratory (ultrasonic) signal.

It is now well known that ultrasonic imaging systems can provide manybenefits in the various analytic arts, such as medicine and the like. Aparticularly beneficial form of ultrasonic imaging utilizes a phasedarray sector scanner (PASS) to sweep a formed beam with the greatestspeed and accuracy. Originally, analog signal processing techniques wereutilized to perform a coherent sum of the various signals receivedacross the plurality N of elements in the front end of the PASS array.That is, it is well known that the accuracy of the beam formation, andparticularly the pointing direction thereof, is directly related to theaccuracy of the phase relationship, or time delays, between the variouselements of the PASS array. It has been shown that a phase accuracy ofabout one part in 32 is necessary to form ultrasonic beams with enoughaccuracy for medical imaging applications. Therefore, each of the timedelays in the PASS array must be adjustable with accuracy at least assmall one-thirty second (1/32) of the time interval required for asingle cycle of the imaging system fundamental frequency. For example,with an imaging system fundamental frequency of about 4.5 MHz., a timedelay accuracy of about 7 nanoseconds is required. Because of thisrequirement, earlier systems have been somewhat replaced by systemsusing baseband signal processing, such as described and claimed in U.S.Pat. No. 4,155,260, issued in 1979, and application Ser. No. 794,095,filed Oct. 31, 1985 now U.S. Pat. No. 4,669,314, both assigned to theassignee of the present application and incorporated herein by referencein their entireties. The baseband processing system is such that phaseaccuracy and time delay accuracy are decoupled from one another, todramatically reduce the requirements on the circuits used for beamformation. That is, the phase characteristics of the basebanddemodulators are controlled so that the phase relationships between theRF signals received at the array elements are preserved after transferto the baseband frequencies. Therefore, the demodulated signals can becoherently summed, which results in a dramatic reduction in the accuracynecessary for the time delays, which are now at the baseband (ratherthan RF) frequencies. However, even with baseband frequency processing,a PASS array front end is: relatively inflexible; sensitive to minorvariations in the properties of analog circuits; and is relativelycostly (as 2N individual analog demodulation circuits and 2N individualand complete time delay sections are required for a N channel array).

A fully digital PASS front end will allow real-time beam formation to becarried out in an accurate, flexible and cost-effective manner. Whilefully digital systems, such as in U.S. Pat. No. 4,324,257 and the like,were first proposed in the 1970s to attempt to overcome some of theinflexibility of analog processing, the fully digital systems described,to date, in the literature have not yet produced beams acceptable formedical imaging applications. The major problem appears to be that thetime delay accuracy of such systems, being determined by the samplingrate of the analog-to-digital converter (ADC) means utilized therein,have typically been an order of magnitude less than the level ofaccuracy needed for medical applications, where the beam is formed ofenergy in the 2-5 MHz. range. That is, the ADC means in such systemshave sample capabilities of between about 10 MHz. and about 20 MHz., sothat resulting time delay accuracies of only between about 100 nsec. andabout 150 nsec. can be obtained, rather than the desired accuracies ofbetween about 6 nsec. and about 15 nsec.

As many other forms of vibratory energy can be used, such as coherentelectromagnetic energy in ladar and radar imaging systems, as well asother types of acoustic energy systems (sonar and the like), it isdesirable to provide beam forming methods and apparatus useful in anysystem for obtaining an image of an object by reflection of an impingentbeam of vibratory energy.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, a method for steering a beam ofvibratory energy to a desired angle θ with respect to the plane of anarray of a plurality N of energy transducers, each assigned to adifferent one of a plurality N of channels and each providing adifferent one V_(j), where 1≦j≦N, of a like plurality N of returnedvibratory energy signals, comprising the steps of: generating aplurality N of sequences each having a multiplicity of pulses of asample strobe S_(j) signal, with each strobe pulse being substantiallyat a time interval T after the preceding strobe pulse, where T is atleast less than the reciprocal of twice the frequency F_(u) of vibratoryexcitation of the transducers; assigning a different one S_(j) of thestrobe signals to each j-th channel; offsetting the commencement time ofthe strobe signal S_(j) in the j-th channel sequence from thecommencement times of strobe signals in all other sequences by a timeinterval t_(Sj) which is a first positive integer multiple M_(j),selected for the each angle θ, of an offset time interval Δt, where Δtis not greater than 1/32nd of the reciprocal of the excitation frequencyF_(u) ; converting, responsive to the strobe signal Sj sequence for thatj-th channel, that one return signal V_(j) directly to a word of digitaldata in an associated j-th one of a like plurality N ofanalog-to-digital conversion (ADC) means; and digitally delaying eachdata word in each of the N channel for a delay time interval t_(dj)selected to cause the summed data words from all N channels to representany change in reflectance in the energy beam at the desired angle θ.

In a presently preferred embodiment of our novel method, the digitaldelaying step includes substeps of: causing the g-th conversion in eachADC means to occur after a time interval t_(Sj) following asystem-conversion synchronization signal; storing each digital data wordfrom the j-th ADC means, in order of conversion, in an associatedlocation of a j-th one of a plurality N of memory means; after thestorage time t_(dj) for that channel, simultaneously reading the g-thdata word from all N memory means; and summing the g-th data wordssimultaneously read from all of the memory means. The channel delay timeinterval t_(dj) can be incremented by an additional amount to focus thearray as the range R increases.

The foregoing invention will be described with particular emphasis toone energy form, e.g. ultrasonic mechanical vibrations, in a presentlypreferred embodiment; it should be understood that this energy form isexemplary and not delimiting.

Accordingly, it is an object of the present invention to provide a novelmethod for forming a vibratory energy beam steered to and focussed at aselected location, with respect to the plane of an array of transducers,by digital manipulation of data converted directly from receivedvibratory energy return signals.

It is another object of the present invention to provide novel apparatusfor forming a vibratory energy beam steered to and focussed at aselected location, with respect to the plane of an array of transducers,by digital manipulation of data converted directly from receivedvibratory energy signals.

These and other objects of the present invention will become apparent tothose skilled in the art upon reading of the following detaileddescription of the invention, when considered in conjunction with theappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the front end of a prior-art PASSvibratory energy (ultrasonic) imaging system;

FIG 1a is a set of time-related graphs of the analog signals availablefrom a subset of the transducer array of prior-art FIG. 1 and of a setof sampling strobe signals utilized therewith;

FIG. 1b is a graph illustrating the coherent sum signal, across thearray aperture, utilizing a uniform sampling function 2, as shown inFIG. 1a, and also of the coherent sum signal utilizing non-uniformdirect sampling of the baseband signal;

FIG. 1c is another set of time-coordinated graphs illustratingnon-uniform sampling for RF channel time delay operation, with themethod of the present invention; and

FIG. 2 is a schematic block diagram illustrating the structure of thefront end signal and logic means, and the associated portions of a mainlogic means, of one presently preferred embodiment of apparatusutilizing the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIGS. 1, 1a and 1b, in a prior art phased arraysector scanning (PASS) vibratory energy (e.g. ultrasonic) imaging system10, a front-end transducer array 11 is comprised of a plurality N ofindividual transducers 11-1 through 11-n, each operating, in atransmission condition, to convert electrical energy of aradio-frequency signal to a desired (e.g. ultrasonic mechanical) form ofvibratory energy and, in a reception condition, to convert reflected(ultrasonic) vibrations to a received RF analog electrical signal, inmanner well known to the art. Other forms of energy, such aselectromagnetic energy (light, radio, etc.) and the like, can be equallyas well utilized. The array is utilized for imaging a spatial location10a which is at a range distance R along a line 10b at some angle θ withrespect to a line 11x normal to the plane of array 11. The vibratory(ultrasonic) signal reflected from point 10a has a wavefront 10b'approaching the plane of array 11 at angle θ. The analog electricalsignals produced at each output of an associated one of the transducersX1-Xn is amplified in one of time-gain controlled amplifier means12a-12n, with gain responsive to the amplitude of a TGC control signal,and is demodulated to a pair of quadrature analog signals jI and jQ,where 1≦j≦n, in an associated pair of demodulation means 13-1a, 13-2a, .. . , 13-na and 13-1b, 13-2b, . . . , 13-nb, responsive to a pair ofquadrature local oscillator signals provided at the ultrasonic frequencyFu from a quadrature means 14. Each analog signal in the basebandquadrature signal pairs 1I/1Q, 2I/2Q, . . . , nI/nQ is individuallysampled and the amplitude thereof converted to a digital data word in anassociated analog-to-digital conversion means 15-1a through 15-na or15-1b through 15-nb. Each conversion data word, formed responsive to oneof a multiplicity of sequential strobe S signal pulses 16a, 16b, . . . ,16n, is then time delayed in an associated one of delay means 17-1athrough 17-na or 17-1b through 17-nb. All conversion data words areindividually delayed, by a delay time established by an associated oneof delay control means 19-j (for that channel j) and thus provides botha delayed baseband I signal jId to a first summer means 18a, and adelayed baseband Q signal jQd to a second summer means 18b. Theresulting coherent sum (RCS) in-phase signal RCSI, at output 10c, andquadration-phase RCSQ, at output 10d, can be operated upon to extractthe amplitude of the return signal from only those objects along theline 10b at the angle θ selected by establishing the channel delayst_(dj) in accordance with the formula t_(dj) =(j-1)(d/V) sin θ, where Vis the velocity of ultrasound propagation in the media immediatelyadjacent the transducers 11 and d is the spacing distance betweenadjacent transducers in the array.

As seen in FIG. 1a, it is well known that the baseband analog signals(e.g. signals 15-1 through 15-4, for the first four in-phase I channels1I-4I), as provided to the ADC means, will have the maxima and minimathereof varying, in the time domain, with relationship (as shown bymaxima line 13x and minima line 13y) which are determined by the spatialsteering angle θ, and with a spacing therebetween determined by thehalf-wavelength of the vibratory (ultrasonic) frequency utilized. If thesector scanning system 10 is utilizing a uniform sampling function S, inwhich all analog signals are sampled essentially simultaneously,responsive to each sample strobe signal 16 (with any one sample strobeseparated from the adjacent sample strobe by a substantially-constantfixed time interval T, which is the reciprocal of the ADC operatingfrequency and at least twice the baseband frequency, but not in excessof the ADC maximum frequency) and the digitized data from eachtransducer is then time delayed, in manner known to the art, then theresulting coherent sum (RCS) signals will only roughly approximatelythat required signal waveform. This can be seen in FIG. 1b, wherein timeis plotted with increasing value along abscissa 20 and the RCS amplitudeis plotted with increasing value along ordinate 21. The amplitude of theresulting coherent sum signal, summed across the array, exists as one ofa plurality of sample values, each indicated by one of X points 22a,22b, . . . 22g, . . . , but only at temporal points (e.g. times t₀, t₄,t₈, t₁₂, t₁₆, t₂₀, t₂₄, . . . ) which are separated from one another bythe substantially constant sampling time interval T. It will seen thatthe resulting coherent sum signal 22 is not particularly accurate, whencompared to the ideal signal curve 23. As taught in the aforementionedapplication, more accurate coherent sum signals, as indicated by the RCSamplitude points 24a, 24b, . . . 24g (shown by the larger circles alongthe curve 24 of FIG. 1b) result if the baseband signals arenon-uniformly sampled.

In accordance with one principle of the present invention, the RF energyresponse signals (e.g. after TGC preamplifiers 12) are directly sampled,without conversion to baseband. Each j-th one of the N analog-to-digital(ADC) converters, which digitizes the RF analog signal Vj from anassociated j-th one of the N transducers of the array, is sequentiallyenabled with a sampling strobe Sj signal that is offset from the strobeof the previous channel by some integer multiple of an aperturecommencement offset, or resolution, time interval Δt. This offset timeinterval is selected to be not greater than a thirty-second (1/32) ofthe reciprocal of the vibratory (ultrasonic) RF frequency F_(u).Therefore, the offset time interval is substantially independent of theinterstrobe time-interval T, established by the Nyquist frequency. Inthe simplest embodiment, each ADC receives a sequential train ofsampling pulses which are themselves all spaced by the sampling timeinterval T apart, with the train of sampling strobe signals between eachpair of adjacent ADCs, in adjacent channels, being "time-slipped" bysome integer multiple M_(j) of the Δt time interval therebetween. Themultiplier M_(j) can be selected for each j-th channel in the samemanner that the channel excitation delay is selected (e.g. for any angleθ and with a known Δt, M_(j) =(j-1)(d/V{t) sin θ).

The method for direct non-uniform sampling of the transducer array RFsignal is illustrated in FIG. 1c. For purposes of illustration, onlyfour of the N transducers are shown; the conversion sample strobe Sjsignal, for each of the four channels (where j=1, 2, . . . k, . . . ,n), has sequential strobe signal pulses separated by the strobe sampleinterval T, but with the channel strobe for a particular reading, in thestrobe sequence of any channel, being separated by the channel offsettime interval Mj·Δt from the strobe for the correspondingly numberedreading for an adjacent channel. The offset time interval Δt is itselfset from a system-wide master clock signal, so that resolution issubstantially equal in all channels. It should be understood that theoffset time interval Δt will best be an integer submultiple of theinterstrobe time interval T; this allows the events in all of theplurality of channels to be established at multiples of a system-wideclock period, relative to one system-wide synchronization event.Illustratively, a synchronization signal 27g will be provided at sometime well prior to the earliest one of the N sampling strobe signals ineach sample-taking set. A number of master clock pulses can be countedto establish the time interval t_(S1) until that time when, for aparticular set of one reading per channel, a first channel strobe pulse26g occurs in the signal S1 for sampling the first RF channel signal V1for the g-th reading set; this occurs, for example, at time t_(a). Eventhough the first channel ADC means strobe signal S1 comprises a train ofsequential strobe pulse signals 26a, 26b, 26c, . . . , 26g, . . . havingthe basic sampling interval T between each pair thereof, the exact timeat which a particular one of the strobe pulses, e.g. pulses 26g, occurs,is set by establishing an initial time-slip interval t_(Sj) after somesynchronization event (generally connected with the excitation sequencefor that beam angle θ then in use). For example, in a system having 20MHz. sampling, the strobe time interval T=(1/20×10⁶)=50 nanoseconds. Foreach set of readings, the strobe signal for the j-th channel occurs witha channel sample time interval t_(Sj) after a system synchronizationpulse 27; that is, each set contains one conversion of data from eachADC means in a then-active channel. Thus, a SYNC pulse 27g occurs tosignal either the start of a first one of a plurality of sets of events(say, the first of 200 sets of sequential conversions in each channel,after one excitation) or the start of each set of events (with eachindividual set of events being one conversion of the RF ultrasonicanalog signal amplitude). In general, each successive channel has alarger channel sample time interval tS_(j) than that of the previouschannel, if θ is negative (as defined in FIG. 1) and clockwise withrespect to normal 11x; the direction of change of this initial channeldelay is reversed if θ is reversed, i.e. if θ is positive andcounter-clockwise (on the opposite side) or normal 11x.

If, for example, a SYNC pulse 27 precedes each set of channel readings,then for the first channel, the after-sync. delay interval t_(S1) (=M₁·Δt) must elapse before the first channel strobe pulse 26g occurs tocall for the data conversion for the one channel 1 conversion in theg-th reading set. Strobe signal S2, to the ADC means in the secondchannel, is a sequence of strobe pulses 28a, 28b, . . . , 28g, . . .each spaced at substantially the same time interval T from the adjacentsignal pulses of the second channel sequence. In the illustrated casewhere the SYNC. pulse precedes each reading set, the g-th strobe pulse28g of this sequence occurs at a channel after-sync. (or sample) timeinterval t_(S2) =(M₂ ·Δt) from the associated SYNC. signal pulse 27gused as the start time in that g-th set. reference. In The k-th channelstrobe Sk signal has the pulses 29a, 29b, . . . , 28g, . . . thereofspaced with substantially the same interval T therebetween, but with theexact time t_(k) of the g-th set data reading pulse 29g occurring at achannel sample time interval t_(Sk), equal to M_(k) ·Δt, after theassociated SYNC pulse 27g. Likewise, the last channel n has strobepulses 30a, 30b, . . . , 30g, . . . with substantially the same timeinterval T therebetween, but with an n-th channel sample time intervalt_(Sn) =M_(n) ·Δt occurring for sample pulse 27g with respect to theSYNC. pulse 27g for that particular set of readings. Reiterating, asynchronization pulse 27g occurs for a g-th set of readings, comprisedof one conversion in each channel, with the actual channel sample strobesignals 26g, 28g, . . . , 29g, . . . , 30g (in the first, second, . . ., k-th, . . . and n-th channels, respectively) occurring with respectivechannel sample time intervals t_(S1), t_(S2), . . . , t_(Sk), . . . ,t_(Sn) after than SYNC. pulse 27g. The duration of each sample timeinterval is M_(j) ·Δt, where M_(j) is determined by the beam steeringangle θ, the array characteristics (e.g. spacing distance d), thevelocity V of propagation of the medium adjacent to the array, etc. andcan be taken from a look-up table and the like, or calculated, asdesired. Each sample time interval is set with a resolution of Δt andcan be adjusted, or "slipped", in either direction, to cause theconversion strobe for that reading set and channel to be within thenearest resolution interval for the set of conversions required to forma beam at the desired angle θ.

Utilizing the novel non-uniform-sampling-at-RF method, it will seen thateach analog-to-digital conversion means need be capable of sampling thereceived "return" vibratory energy (ultrasonic) signal at a rate whichmay be as low as the Nyquist rate (although the sample rate is usuallyseveral times greater), as long as the "aperture jitter", or sampleinitiation resolution, time is less than the desired submultiple (here,a factor of 32) of the intersample time interval T, so as to achieve thenecessary phase accuracy. Thus, any ADC means with at least a 9 MHz.sampling speed and with a sample strobe aperture uncertainty of lessthan about 6 nanoseconds, can be utilized for an ultrasonic system frontend in which a 4.5 MHz. excitation frequency is used.

Again referring to FIG. 1c, it will be seen that the converted data wordin the j-th channel is then time delayed by a channel delay time t_(dj).This delay time is individually established for each individual j-thchannel, by another integer multiple L_(j) of offset interval Δt. Thus,any channel time delay (or delay sequence) can be produced if thesampling function for each channel element is controlled independently.That is, the sampling function for any given element of the ultrasonicarray will be identical to the phasing schedule for the same channelelement in a conventional baseband phased array sector scanning system(such as that in the previously cited U.S. Pat. No. 4,155,260).Therefore, a channel time delay can indeed be provided independent ofthe sampling time for that channel, and responsive only to the arrayparameter j and the steering angle θ(and the range R, if auto-focus isto be performed). For example, the first channel time delay t_(d1) isthe interval between a particular channel 1 sample strobe for a givenset of channel readings, e.g. strobe 26g for the g-th set, and the nextall-channel delay termination, or data-read, R_(d) signal 27'g at whichall N data words in that g-th data set are simultaneously made availablefor forming the RCS for that set. The various channel delay timeintervals t_(dj), where each is the time from the associated strobepulse (e.g. pulses 26g, 28g, 29g, . . . , 30g for the first, second, . .. , k-th, . . . , n-th channels) to the all-channel read pulse, are eachresoluble to time interval Δt. It will be understood that dynamicapodization is easily accomplished by initially sampling only thatplurality P_(min) of channels, where P≦N, then needed in the array (withthe P_(min) channels placed symmetrically about the center of the array)for focussing the beam at nearer points on steering line 10b; additionalsymmetrically disposed pairs of channels will be enabled as needed forfocussing at points with greater range R. As the range increases thedelay time interval td_(p) will be "bumped", or changed (by integermultiples P_(p) of the offset interval Δt), to accommodate changes inthe time delay. It will be seen that the sum of the channel times isconstant, e.g. t_(Sj) +t_(dj) =k (the time between any SYNC. pulse 27gand the read pulse 27' for that g-th set of data readings).

Referring now to FIG. 2, a presently preferred embodiment of our novelphased array sector scanner front end 10' has a transducer array 11 witha plurality N of channel transducers 11a-11n. Even if apodization isused, some centrally-disposed minimum number P_(min) of thesetransducers are always excited. The circuitry for exciting thetransducers to produce a vibratory energy signal is well known and isnot illustrated, for purpose of simplicity. For purposes ofillustration, N may be 64.

The received "return" signal output from each transducer is operatedupon by an associated time-dependent gain-controlledpreamplifier/amplifier (TGC) means 12. Thus, the analog signal outputfrom first transducer 11a is amplified by TGC means 12a, while secondchannel transducer 11b has its analog output signal amplified by TGCmeans 12b, and so forth. The gain of all channels is set by a common TGCcontrol signal (not shown). In accordance with the invention, theamplified RF signal in each channel is directly applied, withoutfrequency conversion or demodulation, to the analog input 32i-1 of thatone of a plurality N of ADC means 32 for that i-th channel, where 1≦i≦N.It will be immediately apparent that this apparatus needs no localoscillator signal generator, no mixer/demodulators and only N (ratherthan 2N) ADC means and delay means, relative to theAD-conversion-at-baseband front end of FIG. 1. In each of the Nchannels, one digitization is carried out responsive to the applicationof an individual strobe signal Si pulse at the conversion-enable input38i-2 of the associated ADC; each ADC is independently storable, withrespect to all other ADC means 32. Responsive to each strobe pulse, aword of output data is provided at a parallel data output port 38-i3.Advantageously, each of the ADC means is substantially identical to anyother one of the ADC means and will have an offset timing (conversioninitiation) accuracy of less than 7 nsec. and will allow analog signalamplitude to be converted to digital data at about 20 megasamples persecond. Illustratively, each of ADC means 32 can digitize the inputanalog ultrasonic signal to an output data word with 7 bits of accuracy,giving the front end and instantaneous dynamic range of greater than 48dB.

The ADC output data word is provided to the data input port 35-ia of theassociated one of a plurality N of first-in-first-out (FIFO) channelread/write (R/W) memory means 35i, each of which is used to establishthe time delay t_(dj) for the associated one of the channels. The inputdata is written into the memory responsive to a memory write strobe Wisignal pulse at a memory write W input; this write pulse occurs slightlyafter each associated strobe Si pulse (the delay being established toaccount for the finite conversion time required for the data to appearat the ADC output after the stobe S pulse is received). The stored datais subsequently provided at the memory means output port 35-ibresponsive to a memory read strobe signal R_(d) pulse at the memorymeans read R input. The memory read R inputs of all of the N FIFO memorymeans are connected in parallel, so that all of the stored singlechannel data readings of a set are read out essentially simultaneously,even though each j-th channel data word is input to the j-th channelmemory individually and in a sequence determined by the non-uniformsampling strobe signal sequence (which is itself determined by channelnumber and angle). Thus, each memory means must have a minimum storagecapacity SCmin, or depth, at least equal to the number of data wordswhich can be provided in the time interval (t_(Si) +t_(di)) intervalbetween the SYNC. pulse 27g and the associated read R_(d) pulse 27'g;thus, SCmin=(t_(Si) +t_(di))/T. Since the total time interval (t_(Si)+t_(di)) and strobe time interval T are predetermined constants, theminimum storage capacity of each memory is also preselectable. Theaddress port (not shown) of the memories can, because of their FIFO modeof operation, be of cyclic style, with the address being changed by eachSYNC. strobe, write or the like pulse. The individual channel samplestrobe S signal and memory write Wi signal (which follows thereafter byat least the ADC means conversion time interval) are provided by anassociated one of a plurality N of individual channel logic means 36i.

The output data words from all channel memory means 35i are added to oneanother in a combiner means 38 to realize the RCS output signal at frontend output 10'z. Combiner means 38 may be a "tree" of adders, such asadders 39 and 40. It is desirable to have an even number N of transducerchannels, whereby a plurality K of 2-input adder means 39a, . . . , 39k(where K=N/2) are used with at least one further level of means 40 forcombining the outputs of the combining means 39, to provide the finaloutput data at output 10'z. If a binary number N of channels are used,where N=2 exp C (C being an integer, e.g., C=6 for N=64), then onlytwo-input combiners, e.g. six levels of two-input combiners 39-40, canbe used in a symmetrical pattern. Such "trees" may have simplifyinginfluence upon the masks necessary to provide in a single integratedsemiconductor circuit the data memory and data-combining means for amultichannel front-end, or portion thereof. Advantageously, the digitalcircuitry of associated channel logic means 36 will be implemented uponthe same I.C.

The channel logic means 36i for each associated front-end channel i,whether of integrated, discrete or other form, comprises a counter anddelay means 42i for providing the sample Si and write Wi signal pulses,responsive to a channel selected clock phase Ci signal and channel logicLi signals. The logic Li signals are provided by an i-th channel logicmeans 45i, responsive to a stored sequence of information (i.e.operating instructions) of which each sequential step is carried outresponsive to some combination of clock pulses after eachsynchronization SYNC signal received; the exact instructional sequencecan, if desired, be modified, responsive to values of θ and/or Rprovided to the front end 10' via an information port 10'p, from asystem central computer means and the like (not shown). In either case,the instructional sequence uses the value of beam steering angle θ toset both the delay interval t_(di) and the sample interval t_(Si) forthe associated channel. The channel logic means 45i provides data to achannel phase φ select means 48i to select a specific one of theplurality Q of different clocks CLKS signal phases as the channel clockphase. Each channel φ select means 48i also receives a plurality Q ofdifferent phases of a high frequency clock CLKS signal. Data fordetermining which phase, number of master clock cycle delay, and similarcharacteristics of the clock C_(i) and logic L_(i) signals, as necessaryfor each i-th channel and dependent upon the value of θ and/or values ofR, as well as channel delay data, can be obtained in any of the plethoraof fashions well known to the art (e.g. look-up tables, down-loadingfrom a central processor, etc.)

The SYNC and CLKS signals are provided by a master control means 50,having a stable oscillator means 52 to provide a master clock signal ata predetermined frequency F_(M) (e.g. 200 MHz.). The master clock signalis provided to one input 54a of a master logic means 54. The masterclock signal is squared-up by a Schmitt-trigger means 56. The triggeroutput signal (a substantially square wave at frequency F_(M)) isapplied to the input 58a of a multiple-stage Johnson counter means 58.The master logic means has another input 54b which is provided from afirst output 58b of the counter means with a clock pulse related to themaster clock signal, e.g. can be as often as one clock pulse for everyclock half-cycle time interval. The Johnson counter also provides, atits second outputs 58c, the plurality Q of separate signals, eachpulsing to a chosen level only once in every Q cycles of the masteroscillator frequency F_(M). Thus, if Q=8, these eight clock CLKS signalsare each separately and mutually-exclusively pulsed at a frequency ofF_(M) /Q=25 MHz. Master logic means 54 provides, inter alia: a series ofthe synchronization injection SYNC signals, at a first output 54c, toinitiate the strobe pulse/memory-write/delay sequence in the variouschannels; and the system common read R_(d) signal pulse, at a secondoutput 54d, to commonly end each conversion/write/delay-till-read cycleof each front end channel.

Referring now to FIGS. 1c and 2, in operation of PASS front end 10',SYNC pulses occur only after termination of each transmission excitationto the transducers of array 11; some additional delay may be added toallow for settling time and other effects. Thus, an initial interval ofsome preselected time, e.g. about 2 microseconds, may be required aftertermination of excitation and before the analog output of the firsttransducer of the array 11a is converted to a digital data word. If theoscillator 52 master clock frequency F_(M) is 200 MHz., this requires400 master clock pulses. Thus, the first transducer sample time intervalt_(S1) (between the synchronization pulse of any set, e.g., the g-th setSYNC pulse 27g, and the first channel strobe S1 pulse for that same set,e.g. the g-th set strobe 26g) is illustratively selected to be at least2 microseconds long. This same "dead time" interval will be utilized atthe start at each of the sample time intervals t_(Sj) for each of the Ntransducer channels. As all stored data for a particular set is to beread simultaneously, responsive to a common read R_(d) pulse 27'g, thetime interval between the synchronization pulse 27g and the read pulse27'g, for that particular reading set, is constant, such that thevariable delay time interval t_(dj) for each channel is subtractedtherefrom to arrive at the sample time t_(Sj) at which the strobe mustoccur after the synchronization pulse. For purposes of illustration, itwill be assumed that the array has parameters such that (d/V)=1microsecond so that the delay time interval t_(di) =(i-1) sin θmicroseconds. For the particular angle θ=-30°, i.e. θ is an angle of 30°clockwise with respect to the array normal (as defined in FIG. 1), thedelay time interval for the i-th channel will be (t_(di),=0.5(i-1)microseconds). The maximum delay interval (t_(di) +t_(Si)) is selectedto allow the shortest channel delay, here t_(dn), to still besufficiently long for all necessary timing to occur. The channel delaytime interval t_(di) will decrease and the channelsynchronization-strobe delay t_(Si) will increase, as the channel numberi increases.

The angle θ data is received at front end data port 10'p prior to anySYNC. pulse 27. Now, after the g-th reading set synchronization pulse27g occurs, the channel one logic means 45a provides logic informationsignals La to counter and delay means 42a, and provides phase selectdata to the select means 48a, to select the proper one of the eightmaster clock phases to supply as the first channel clock Ca signal tocounter and delay means 42a. This clock phase, after being counted for anumber of occurrences established by the data of one of signals La,causes the first channel strobe signal S1 pulse 26g to occur at timet_(a). Illustratively, with the first strobe time interval t_(S1) beingequal to 2.000 microseconds (the initial 2 microsecond dead time plus 0additional strobe delay time for the first array channel, with i=1), thefirst channel logic means 45a will have calculated that: (a) themultiple M₁ =400 master clock cycles need be counted, with the firstphase of the CLKS signal being utilized for clock signal Ca; and (b)that the M₁ /Q=50th occurrence of clock signal Ca, after thesynchronization signal, should cause the first strobe S1 pulse 26g to bepresent at the first channel ADC means 32a. The associated channelmemory write signal, e.g. first channel write signal W1, occurs notlater than the next clock Ca pulse after the strobe; this delay can beset (by use of gate delays, counting of master clock pulses, or thelike) to be at least the duration of the conversion time and less thanthe time to occurrence of the next strobe S_(i) for that channel.Thereafter, the first channel delay time t_(d1) =t_(d) max(illustratively preselected to be 35 μsec.), so that the sum (t_(di)+t_(Si)) is constant at 37 μsec (taking 7400 master clock cycles). Thegeneral formulae are: t_(di) =(35-(i-1)/2) microseconds; and t_(Si)=(2+(i-1)/2) microseconds. Thus, delay interval t_(d1) =35μ (equivalentto 7000 master clock cycles), as t_(S1) was previously selected as 2μsec (400 clock cycles). For the second channel, with i=2, t_(d1)=35-1/2=34.5 μsec. (or 6900 clock cycles), and t_(S1) =37-34.5=2.5 μsec(or 500 clock cycles) to be counted in second channel counter and delaymeans 42b. In the last (N=64) channel, t_(d64) =35-63/2=3.5 μsec (or 700clock cycles) and t_(S64) =37-3.5 μsec. (or 6700 clock cycles). When7400 master clock cycles have been counted (in master clock means 54)after the SYNC. pulse 27g, the g-th set all-channel memory read-outpulse 27'g is provided. It will be seen that dynamic focus time slippagecan be easily implemented, given entry of range R information, by anappropriate increase in any channel's delay interval t_(di) (where theincrease is (t)_(focus) =(a² /2RV)(1-(x_(i) /a)²) cos² θ, with x_(i)being the distance from the array center to the center of the i-thtransducer, and a being the maximum x_(i) distance for that array).Small changes in time delay due to dynamic focus effects can be easilyimplemented by changing the clock phase, selected by the associatedselect means 48, as a function of range R. A similar decrease will occurin time interval t_(Si).

While only certain preferred features of the invention have been shownby way of illustration, many modifications and changes will occur tothose skilled in the art. For example, the RF return signal(s) can befrequency-converted to an intermediate (IF) frequency prior toconversion to digital data; we still consider the signals to be at an RFfrequency, rather than at baseband frequencies. It is to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit and scope of the invention.

What we claim is:
 1. A method for steering a beam of vibratory energy,at an excitation frequency F_(u), to a desired angle θ with respect tothe normal to the plane of an array of a plurality N of transducers,each assigned to a different j-th one of a plurality N of channels,where 1≦j≦N, comprising the steps of:(a) generating for each j-thchannel a different sample strobe S_(j) signal sequence having amultiplicity of sequential pulses, with any pair of pulses of thatchannel strobe sequence having substantially a time interval Ttherebetween, where T is at least less than the reciprocal of twice theexcitation frequency F_(u) ; (b) offsetting, from the occurrence of eachof a sequential set of synchronization signal pulses, a commencementtime of a next pulse of the strobe S_(j) signal sequence in the j-thchannel essentially by a sample time interval t_(SJ) which is a firstpositive integer channel multiple M_(j), selected for each angle θ, ofan offset time interval Δt, where Δt is less than T and is a fixedfraction of the reciprocal of the excitation frequency F_(u) ; (c)maintaining all strobe S_(j) signal pulses, after the offset next pulse,with substantially the time interval T therebetween until anotheroffsetting step occurs; (d) converting, responsive to each pulse of thestrobe S_(j) signal sequence, a j-th channel return signal V_(j)directly to a word of digital data representing a present amplitude ofthe return signal in that j-th channel; (e) digital delaying each dataword in each of the N channels for a channel delay time interval t_(dj)which is a second positive integer channel multiple P_(j), selected foreach angle θ, of the offset time interval Δt; and (f) coherently summingresponsive to a signal at a substantially fixed time interval after eachsynchronization signal, each delayed data word then simultaneouslyavailable from all N channels, to obtain each point of data representinga reflectance of the energy beam at the desired angle θ.
 2. The methodof claim 1, wherein step (d) includes the step of providing anassociated j-th one of a like plurality N of analog-to-digitalconversion (ADC) means in which the conversion of the V_(j) signaloccurs responsive to each strobe S_(j) signal pulse; and step (e)includes the steps of: (e1) sequentially storing each sequential digitaldata word from the j-th ADC means, in order of conversion, in anassociated location of a j-th one of a plurality N of memory means; (e2)after the delay interval t_(dj) for that channel, sequentially readingone data word from each of the N delayed memory means; and (e3) summingall of the N data words substantially simultaneously read from all ofthe N memory means to obtain the reflectance data.
 3. The method ofclaim 2, wherein there are an even number N of channels, and step (e3)includes the steps of: (e3a) substantially simultaneously summing eachone of N/2 different pairs of delayed data words to obtain a first-levelsummed data word; and (e3b) then substantially summing all of the N/2resulting first-level summed data words.
 4. The method of claim 3,wherein N=2^(C), where C is a positive integer greater than 1, and step(e3b) includes the steps of: further summing, at each of (C-1)additional levels, each different pair of summed data words resultingfrom a previous summing level, to arrive at a single final data wordsum.
 5. The method of claim 2, wherein step (e2) further includes thestep of providing a single system-wide read R_(d) signal having amultiplicity of pulses, each for causing each of the N memory means tooutput one stored data word.
 6. The method of claim 5, wherein step (e2)further includes the step of causing each pulse of the read R_(d) signalto occur at a fixed time interval after the occurrence of an associatedone of the synchronization signal pulses.
 7. The method of claim 6,wherein step (e2) further includes the step of setting the fixed timeinterval to be not less than (t_(Sj) +t_(dj)).
 8. The method of claim 2,wherein step (e1) includes the step of providing a memory write W_(j)signal to the j-th memory means, during a time interval, less than thetime interval T, after each conversion strobe pulse in that j-thchannel, to cause storage of the data word then presented to that memorymeans from the associated j-th channel ADC means.
 9. The method of claim1, wherein step (b) includes the step of selecting the fixed fraction tobe not greater than 1/32.
 10. The method of claim 1, wherein step (b)includes the step of selecting the fixed fraction to be equal to 2^(-x),wherein the integer x is at least
 5. 11. The method of claim 1, whereinstep (b) includes the step of adjusting the offset commencement time ofat least one sample time interval t_(Sj) by another integer multiple ofoffset time interval Δt to dynamically focus the beam with changes inrange R.
 12. The apparatus of claim 1, wherein the vibratory engery isultrasonic energy.
 13. Front-end receiving apparatus for a coherentimaging system having a sector-scanning phased array of a plurality N oftransducers, each different one in a different j-th channel, wherein1≦j≦N, and providing a received signal, at a vibratory energy excitationfrequency F_(u), of a amplitude responsive to the reflectance of mediaat a range R and a steering angle θ with respect to the array normal,comprising:each j-th different one of a plurality N of analog-to-digitalconverter (ADC) means for directly converting the instantaneousamplitude of a j-th channel RF frequency signal F_(j) to a digital dataword responsive to each pulse of a strobe S_(j) signal for that j-thchannel; each different j-th one of a plurality N of FIFO memory meansfor storing, responsive to each pulse of a j-th channel write W_(j)signal, and for presenting, in FIFO order, a next sequential stored dataword responsive to each pulse of a read R_(d) signal; each differentj-th one of a plurality N of channel logic means for independentlygenerating both (a) the j-th channel strobe S_(j) signal, with amultiplicity of sequential pulses each separated from the adjacentpulses by a time interval T, where T is at least less than thereciprocal of twice the transducer excitation frequency F_(u), and witha next strobe pulse, after each of a plurality of synchronization signalpulses, being offset from that associated synchronization signal pulseby a time interval t_(Sj) which is a first positive integer channelmultiple M_(j), selected for each channel and each angle θ, of an offsettime interval Δt, where Δt is less than T and is a fixed fraction of thereciprocal of the excitation frequency F_(u), but with all strobe S_(j)signal pulses, after that next offset strobe pulse, being maintainedwith substantially the time interval T therebetween until anothersynchronization pulse occurs; and (b) the j-th channel write W_(j)signal at a time after each strobe pulse and prior to a next sequentialstrobe pulse for that j-th channel; master logic means for generatingeach of a sequence of said read R_(d) signal pulses at a time to causeeach of the plurality N of presented stored data words to have beendelayed in storage by a time interval t_(dj) which is a second positiveinteger multiple P_(j), also selected for each channel and each angle θ,of the offset time interval Δt; and means for combining the data of allN data words substantially simultaneously presented at the N memorymeans, responsive to each read R_(d) signal pulse, to output a word ofcoherently summated data from the front end.
 14. The apparatus of claim13, where N is even and said combining means includes a plurality N/2 ofmeans each for combining the data words output from two different onesof said N memory means.
 15. The apparatus of claim 14, wherein N=2^(C),where C is a positive integer greater than 1, and said combining meansfurther comprises a binary tree formation of (C-1) additional levels ofmeans for combining two different ones of data words output from thecombining means of the immediately higher level; with an output dataword from the single combining means of a lowest (C-th) level being thefront-end output data word.
 16. The apparatus of claim 13, wherein thefixed fraction is selected to be not greater than 1/32.
 17. Theapparatus of claim 13, wherein the fixed fraction is selected to beequal to 2^(-x), where the integer x is at least
 5. 18. The apparatus ofclaim 13, wherein the master logic means comprises means for poviding aplurality of clock signals, each at a different phase of the samefrequency; and each of the N channel logic means comprises means forselecting one of the plurality of clock signal phases and for countingeach cycle occurrence thereon for generating each strobe S_(j) signalfor that j-th channel.
 19. The apparatus of claim 18, wherein the masterlogic means further comprises means for providing each synchronizationsignal at a time interval, prior to each read R_(d) signal pulse,substantially equal to a selected constant for each particular angle θ.20. The apparatus of claim 13, wherein each j-th channel logic meansincludes means for providing, responsive to externally-provided angle θdata, values of the multiples M_(j) and P_(j).
 21. The apparatus ofclaim 20, wherein each j-th channel logic means also receivesexternally-provided range R data; and further comprises means forvarying a selected sample clock phase to cause the then-active channelsof the front-end to be correctly dynamically focussed with varyingrange.
 22. The apparatus of claim 13, wherein said master logic meansincludes means for controlling the time at which at least one pair ofthe channels are enabled to actively participate in the array, to usedynamic apodization to reduce error when the range R is less than apreselected distance.
 23. The apparatus of claim 13, wherein thevibrating energy is ultrasonic energy.